Wide band amplifier



April 29, 1941. J. A. RANKIN WIDE BAND AMPLIFIER Filed Oct. 1, 1938706/210 0F NEXT/IMPL.

F 70 SIGNAL SOURCE INV EN TOR. JO/{N A. RANKIN Patented Apr. 29, 1941stares aren't FFICE WIDE BAND ARIPLIFIER John A. Rankin, JacksonHeights, N. Y., assignor to Radio Corporation of America, a commation ofDelaware Application October 1, 1938, Serial No. 232,711

2 Claims.

amplifier response curve; the cancellation-being 3 eifected by feedingback from the output of the amplifier to its input a properly phasedalternating voltage developed by the output alternating current across apath including a re sistor in series with the primary of a feedbacktransformer, shunted across the output load; and the mutual inductancebetween the coils of the transformer imparting the correct phase to thefeedback voltage.

Another object of my invention is to derive from the output circuit of aresistance coupled amplifier an alternating voltage which is in phasequadrature with the input signal voltage thereby to reduce the effect ofthe output capacity on the frequency-response curve of the amplifier.

Still other objects are substantially to improve the efficiency andoperation of resistancecoupled amplifiers, and more especially toprovide the latter with substantially flat response characteristics.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawing:

Fig. 1 is a. circuit embodying the invention,

Fig. 2 shows a modification.

Referring now to the drawing, wherein like reference characters in thetwo figures indicate similar circuit elements, there is shown anamplifier tube l of any desired type between whose input electrodes 2and 3 is connected a source of signal voltage. The latter may be a prioramplifier stage of a resistance-coupled amplifier; in general, thesource of signal voltage may be of any type. Assuming for the presentcase that the tube l is included in a multistage resistance-coupledamplifier, those skilled in the art know that plate 4 is connected tothe +13 terminal of a direct current voltage source through the loadresistor R. The capacity 5 bypasses the currents of signal frequenciesto ground. The cathode 3 is connected to ground through the customarygrid bias resistor-shunt capacity network ii; grid leak resistor 8provides the biasing path for grid 2. The reference character C denotesthe output shunt capacities, shown in dotted line as a lumped capacityacross the load R. It is to be understood that the symbol C represents alumped capacity made up of the output capacity of tube I, the inputcapacity of the next tube and stray circuit capacities.

The signal voltage developed across load R, and in amplified form, istransmitted by coupling capacity 1 to the grid of the followingamplifier. The latter may be of the same type as amplifier l. Theconstruction of resistancecoupled amplifiers of necessity depends on thefrequency band which must be covered. In television work, where a widefrequency band is necessary for proper transmission of the radiofrequencies, it is difficult to maintain a fiat amplitude-frequencycharacteristic without large reduction in gain per stage. Thefundamental cause of a drop in the amplitude; response ofresistance-coupled amplifiers at high frequencies is the fact that theshunt capacities C are unavoidable. Many arrangements have, therefore,been proposed to boost the response curve at the high frequency end tocompensate for the drop in gain due to C.

According to the present invention the current bypassed by the element 0is supplied by a feedback path comprising resistor R2 in series with theprimary winding L1 of the transformer M. The resistor R2 and transformerM are effectively in series, and are shunted across the load resistor R.The secondary winding L2 of transformer M is connected by lead 9 to thegrid circuit of the amplifier I. There is, in general, developed acrossresistor R2 an alternating voltage which is fed back across M (negativein polarity sign) and lead 9 to the amplifier input. The effect of thisfeed back voltage is to cancel out the shunting effect of capacity C. Inother words, there is fed back across a -M an alternating voltage inquadrature with the signal input voltage thereby to cancel out C.

It is not believed necessary to complicate this description with amathematical analysis. It is believed suificient, for those skilled inthe art,

to state that fiat amplitude-frequency response may be secured for suchfrequencies where the magnitude of R2 greatly exceeds (ywL) so that thecurrent through L1 is directly proportional to plate volts. By way ofillustration the coil L1 may have a value of the order of 30microhenrys; and the frequency band in megacycles should be of the orderof 3 mo. For a type 1851 tube, for example, a gain of 90, for a bandwidth of 3.2 mc., can be secured.

Fig. 2 shows a modified circuit arrangement wherein the coil L2 isincluded in series between the grid condenser and the grid 2'. The gridleak resistor 8 is grounded in this case. In the modified arrangementthere has been eliminated from Fig. 1 the effect of the voltage divideraction of the grid leak and the output impedance of the prior stage.This circuit arrangement functions, otherwise, in the manner explainedin connection with Fig. 1. In both arrangements the improvement in gainover a conventional wide band amplifier, with flat response to the sameupper limiting frequency, is of the order of five to one.

While I have indicated and described several systems for carrying myinvention into effect, it 'will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but

,that many modifications may be made without departing from the scope ofmy invention, as set forth in the appended claims.

What I claim is:

1. In a wide band signal amplifier of the type comprising at least oneamplifier tube provided with a resistance coupling element in itsoutput; means for compensating the efiect of the inherent outputcapacities of the tube, said means comprising an inductive elementarranged in series with said resistance element, a second inductiveelement connected between said tube input electrodes, said inductiveelements being magnetically coupled to provide a negative mutualinductance, and said resistance element and mutual inductance developinga signal voltage which is applied to said input electrodes in phasequadrature with the signal input voltage thereby to cancel out theeiiect of said inherent capacities.

2. In a signal amplifier of the resistancecoupled type havingunavoidable output capacities which lower the high frequency end of theresponse curve, said amplifier having input and output circuits, afeedback path between the output and input circuits for boosting saidcurve end, said path comprising a resistance element across the outputcircuit, a coil in series with the resistance element, a second coilconnected to said input circuit, said two coils being magneticallycoupled in such a manner as to provide negative mutual inductancewhereby signal voltage developed across the resistance element is fedback to said input circuit in phase quadrature with the signal inputvoltage.

JOHN A. RANKIN.

